1. Technical Field of the Invention
The present invention generally relates to cell detection by terminals in mobile radio systems which use at least two synchronization signals for base station synchronization.
2. Description of Related Art
Code division multiple access (CDMA) communication technologies were developed to allow for multiple users to share scarce radio communication resources. In the general CDMA method, each user terminal is assigned a unique code sequence to be used to encode its information signal. A receiver, knowing the code sequences of the user terminal, can decode the received signal to reproduce the original information signal. The use of the unique code sequence during modulation provides for an enlarging of the spectrum of the transmitted signal resulting in a spread spectrum signal. The spectral spreading of the transmitted signal gives rise to the multiple access capability of CDMA.
If multiple users transmit spread spectrum signals at the same time, the receiver will still be able to distinguish a particular user's signal, provided that each user has a unique code and the cross-correlation between codes is sufficiently low. Ideally, the cross-correlation should be zero, i.e., the codes should be orthogonal in the code space. Correlating a received signal with a code signal from a particular user will result in the despreading of the information signal from that particular user, while signals from other users will remain spread out over the channel bandwidth.
An improvement of the CDMA method, known as Wideband CDMA (WCDMA), has been developed by a number of organizations around the world. One of the most popular of these WCDMA efforts is that of the Third Generation Partnership Project (3GPP). Some of the benefits provided by WCDMA include support for increased bandwidth and bitrates, and provisions for packet data communication and other services. In a WCDMA system, a mobile terminal, referred to as user equipment (UE), communicates with one or more base stations each serving a particular cell.
An important consideration for mobile terminals in a communication system, such as a WCDMA system, is that of standby time. To increase standby time the power consumption when the terminal is turned on, but not in use, should be minimized. Power consumption can be reduced by powering off parts of the terminal, such as signal processing circuitry, when it is not in use. However, the primary factor that limits standby time in a terminal is the need for the terminal to monitor for signals, such as paging messages, and perform signal measurements, such as signal strength measurements of nearby cells, during standby. Another important, but power demanding procedure is that of cell search or cell detection. This procedure includes searching for new cells close to the terminal. False cell detection results in the unnecessary powering up of mobile terminal circuitry, which contributes to increased power consumption and reduced standby time. The present invention provides for an improved method for cell detection which reduces the false detection rate and improves detection performance. In addition, a longer standby time for the terminal can be achieved.